1. Field of the Invention
The present invention relates generally to a system for controlling the exhaust emissions from an internal combustion engine and, more specifically, to a system for selectively reducing nitric oxides while the engine is operating in regimes that produce large quantities of nitric oxides. More specifically, but without restriction to the particular embodiment and/or use which is shown and described for the purposes of illustration, the present relates to a system for selectively reducing nitric oxides while the engine is operating in a lean burn regime.
2. Discussion of the Related Art
In recent years there has been an increase in the utilization of engines that have the ability to run in a lean-burn regime of operation, this is due to the inherently higher efficiency and hence lower carbon dioxide emissions during this type of operation. Internal combustion engines, both compression and spark ignition, burn various types of fuel with air and produce many species of products. The environmentally undesirable products have been attracting a great deal of attention. The typical exhaust stream of an internal combustion engine consists of appreciable quantities of the following chemical species: unburned hydrocarbons (HC), carbon dioxide (CO.sub.2), carbon monoxide (CO), nitrogen (N.sub.2), nitrogen monoxide (NO), nitrogen dioxide (NO.sub.2) and un-reacted oxygen (O.sub.2). Modern automobiles are equipped with catalytic converters that function to reduce the amounts of undesirable species, namely, NO.sub.2, NO, HC, and CO. NO.sub.2 and NO are both commonly referred to as NOx.
Internal combustion engines, in an effort to become more efficient, are attempting to implement lean-burn cycles. During low power demands on the engine, the engine runs lean, meaning that there is a less than a stoichiometric amount of fuel for the amount of air present. This concept increases engine efficiency by consuming less fuel. Unfortunately, this regime leads to the increased production of NOx by-products. NOx by-products are formed during the high temperature reaction between nitrogen and oxygen. It is difficult to eliminate this condition in the periods of lean-burn operation, due to the fact that excessive amounts of both oxygen and nitrogen will be present when an engine is running lean.
The traditional method of treating exhaust pollutants is to employ some type of catalysis system. The typical catalysis in today's automobile engine is designed to treat HC, CO, and NOx. This type of catalyst is termed a three-way catalyst. Most catalysts utilize precious metals deposited on a substrate. The most common precious metals employed in automotive catalytic converters are platinum, palladium, and rhodium. During lean operating conditions this type of catalyst does an adequate job facilitating the following oxidation reactions: EQU 4HC+5O2.fwdarw.4CO2+2H2O EQU 2CO+O2.fwdarw.2CO2
These reactions are easily preformed due to the excessive amounts of O2 available during a lean-burn type of operating condition. The difficulty in the lean burn regime is dealing with the reduction of NOx. This is a basic chemistry problem that has been facing the automotive industry for many years. A great deal of time and research has been devoted to developing a lean NOx catalyst. Currently, lean NOx catalysts have a lower operating temperature than the conventional catalyst that aids in the oxidation of HC and CO making lean NOx catalysts incompatible with automotive aftertreatment systems. Lean NOx catalysts have not been able to selectively reduce NOx to a level that is acceptable to the automotive industry.
There is, therefore, a need to provide to selectively reduce NOx while an engine is operating in a lean burn regime.